The present invention relates to a method of recovering valuable metals from IZO scrap such as a spent indium-zinc oxide (IZO) sputtering target or IZO mill ends arising during the manufacture of such a target (hereinafter collectively referred to as the “IZO scrap” in this specification). Incidentally, “recovery of valuable metals” as used herein shall include the recovery of compounds of oxides, hydroxides and the like, which have valuable metal as their component.
In recent years, an indium-zinc oxide (In2O3—ZnO: generally known as IZO) sputtering target is being widely used for a transparent conductive thin film of an LCD device and the like. In many cases, however, a thin film is formed on a substrate or the like by employing the sputtering method as the thin-film forming means.
Although the sputtering method as the thin-film forming means is a superior method; for example, if a sputtering target is used to form a transparent conductive thin film, the target will not be consumed uniformly. A portion of the target with severe consumption is generally referred to as an eroded portion, and the sputtering operation is continued until immediately before a backing plate supporting the target becomes exposed due to the further consumption of the eroded portion. This target is subsequently replaced with a new target.
Accordingly, a spent sputtering target will have numerous non-eroded portions; that is, unused portions of the target, and all of these portions become scrap. Moreover, even during the manufacture of an IZO sputtering target, scraps (mill ends) will arise from grinding dust and cutting wastage. Generally speaking, the scrap contains zinc oxide (ZnO) at approximately 10.7 wt %, but the scrap is mostly indium oxide (In2O3).
Because high-purity materials are used as the IZO sputtering target material and because indium is particularly expensive; in general, indium and zinc are simultaneously recovered from the foregoing scrap materials. As this kind of indium recovery method, conventionally, a method that combines wet refining such as the acid solution process, ion exchange method and solvent extraction method has been used.
For instance, there is a method of subjecting an IZO scrap to cleansing and pulverizing, dissolving the obtained material with hydrochloric acid, precipitating and eliminating impurities, such as zinc, lead and copper, as sulfide by passing hydrogen sulfide through the solution, thereafter adding ammonia to neutralize the solution, and recovering the resulting indium hydroxide.
Nevertheless, the indium hydroxide obtained by the foregoing method has inferior filtration property, requires long operational process, and contains large amounts of impurities such as Si and Al. In addition, with the created indium hydroxide, since the grain size and grain size distribution will vary depending on the neutralization condition, maturization condition and other conditions, there is a problem in that the characteristics of the IZO target cannot be stably maintained upon subsequently manufacturing such IZO target.
Conventional technology (similar technology) and its advantages and disadvantages are described below.
As one example of such conventional technology, there is an etching method of a transparent conductive film including the steps of reducing an ITO film deposited on a substrate by an electrochemical reaction in the electrolyte, and dissolving the reduced transparent conductive film in the electrolyte (refer to Patent Document 1). However, the object of this method is to obtain a mask pattern with high precision, and relates to technology that is different from the recovery method.
For pretreatment for recovering valuable metals from ITO, there is also technology of isolating, in the electrolyte, the impurities contained in an In-based brazing filler material used in the bonding with the backing plate (refer to Patent Document 2). Nevertheless, this method does not relate to direct technology of recovering valuable metals from ITO.
Moreover, upon recovering indium from an intermediate obtained as a by-product of the zinc refining process or from an ITO scrap, disclosed is technology of separating tin as halogenated stannate, performing reduction treatment with hydrochloric acid or nitric acid solution, subsequently adjusting the pH of this aqueous solution to be between 2 and 5, reducing metallic ions of iron, zinc, copper, thallium and the like in order to make the metallic ions into a substance that will not precipitate easily, and isolating the indium component in the aqueous solution (refer to Patent Document 3). With this technology, however, there are problems in that the refining process is complicated and a superior refining effect cannot be expected.
Further, as a method of recovering high-purity indium, disclosed is technology of dissolving ITO with hydrochloric acid, adding alkali thereto to make the pH within a range of 0.5 to 4, eliminating tin as hydroxide, subsequently blowing hydrogen sulfide gas in order to eliminate hazardous materials such as copper and lead as sulfide, and electrowinning indium metal by performing electrolysis using the obtained solution (refer to Patent Document 4). There is a problem with this technology in that the refining process is complicated.
In addition, proposed is a method of dissolving an ITO indium-containing scrap with hydrochloric acid to obtain an indium chloride solution, adding a sodium hydroxide solution to this solution to eliminate tin as tin hydroxide, additionally adding a sodium hydroxide solution thereto after the elimination to obtain indium hydroxide, filtering the obtained indium hydroxide to obtain indium sulfate from the filtered indium hydroxide, and obtaining indium by electrowinning with the indium sulfate (refer to Patent Document 5). Although this is an effective method with a significant refining effect, there is a drawback in that the process is complicated.
Also proposed is a method of recovering indium including the steps of dissolving an ITO indium-containing scrap with hydrochloric acid to obtain an indium chloride solution, adding a sodium hydroxide solution to the indium chloride solution to eliminate tin contained in the scrap as tin hydroxide, substituting indium with zinc from the solution after eliminating the tin hydroxide, and thereafter recovering indium (refer to Patent Document 6). Although this is also an effective method with a significant refining effect, there is a drawback in that the process is complicated.
Additionally disclosed is a method of recovering metallic indium including the steps of extracting suboxide-containing cast scrap floating on molten metallic indium, introducing this into an atmosphere furnace, introducing argon gas into the furnace after vacuating the furnace once, heating the furnace to a prescribed temperature, and reducing the suboxide-containing cast scrap (refer to Patent Document 7).
Although this is in itself an effective method, there is a drawback in that this is not a fundamental recovery method from IZO scrap. In light of the above, a method that is efficient and with a versatile recovery process is being sought.    [Patent Document 1] Japanese Patent Laid-Open Publication No. S62-290900    [Patent Document 2] Japanese Patent Laid-Open Publication No. H8-41560    [Patent Document 3] Japanese Patent Laid-Open Publication No. H3-82720    [Patent Document 4] Japanese Patent Laid-Open Publication No. 2000-169991    [Patent Document 5] Japanese Patent Laid-Open Publication No. 2002-69684    [Patent Document 6] Japanese Patent Laid-Open Publication No. 2002-69544    [Patent Document 7] Japanese Patent Laid-Open Publication No. 2002-241865